It is becoming easier to tamper with digital images in ways that are difficult to detect .In this work a semi-fragile watermarking scheme is presented. This scheme is applied to digital color image in spatial domain. The image is divided into parts, each part has its authentication mark embedded in it, It would be able to be insure which parts of the image are authentic and which parts have been modified.First objective is to find the best number of partitions; this achieved by increasing the number of partitions. But Peak signal to noise ratio (PSNR) measure should be kept in a predefined quality range, while tuning the watermark strength parameter.Second objective is to find the best least significant bits (LSB’s) positions, because one bit of authentication mark is embedded into one of first four LSB's according to embedding function to increase secrecy.Third objective is to find the most suitable color model for the proposed system. It has been tested the most commonly used color models; RGB, YUV, YIQ, YCbCr1 and YCbCr2.In this work it has been taken into consideration the value of errors that generated during transformations among color models. The results show YUV color model is the best for information hiding in color images.The authentication process carried out without need for the original image (Blind method) .The results show the quality of the watermarked image remains very good by keeping Mean Square Error (MSE) and (PSNR) in an acceptable quality range. The proposed watermark survives some types of unintended modification such as familiar compression software like (WINRAR and ZIP) with a high ratio. Also it survives some types of image transformation format like (TIFF, PICT, and PDF).
